Hydrocarbons and NOx emissions are a direct result of the combustion process in an internal combustion engine. To reduce such harmful emissions, catalytic converters are employed to reduce their toxicity. For gasoline engines, “three-way catalysts” are used to reduce nitrogen oxides to both nitrogen and oxygen, as shown by the equation below:2NOx→xO2+N2 
These three-way catalysts are also used to oxidize carbon monoxide to carbon dioxide, which is shown by the second equation below:2CO+O2→2CO2 
Furthermore, these three-way catalysts are also used to oxidize hydrocarbons into carbon dioxide and water, as shown by the third equation below:CxHy+nO2→xCO2+mH2O
In the case of an engine which uses compression ignition, such as a diesel engine, the most commonly employed catalytic converter is the diesel oxidation catalyst. This catalyst employs excess O2 in the exhaust gas stream to oxidize carbon monoxide to carbon dioxide and hydrocarbons to water and carbon dioxide. These converters virtually eliminate the typical odors associated with diesel engines, and reduce visible particulates; however they are not effective in reducing the NOx due to excess oxygen in the exhaust gas stream.
One way of reducing NOx emissions in a diesel engine utilizes a Selective Catalytic Reduction (SCR) Catalyst in the presence of a reducing agent such as ammonia (NH3) to modify the engine exhaust. Existing technologies utilize SCR and NOx traps or NOx absorbers. The ammonia is typically stored on board a vehicle either in pure form, either as a liquid or gas, or in a bound form that is split hydrolytically to release the ammonia into the system.
An aqueous solution of urea is also commonly used as a reducing agent. The urea is stored in a reducing tank that associated with the system. A dosing valve disposed on the exhaust manifold upstream of a catalytic converter meters the delivery of a selected quantity of urea into the exhaust stream. When the urea is introduced into the high temperature exhaust, it is converted to a gaseous phase and the ammonia is released to facilitate reduction of NOx. In lieu of ammonia, diesel fuel from the vehicle's fuel supply can be used as the reducing agent. In this expedient, a quantity of diesel fuel is administered directly into the exhaust via the dosing valve.
Additionally, particulate-specific traps accumulate unburned hydrocarbons, and dehydrogenated material is not removed by combusting a reducing agent such as diesel fuel to supply heat to oxidize or burn off these materials, this results in the trap reducing exhaust flow and increasing exhaust back pressure on the engine cylinders, reducing engine efficiency.
In either case, a dosing valve assembly is mounted directly on the exhaust manifold, and thus operates in a very high temperature environment that can reach temperatures as high as six-hundred degrees Celsius. Accordingly, the dosing valve is cooled to prevent decomposition or crystallization of the urea, or coking due to failure of diesel fuel reducing agent prior to delivery into the exhaust stream, and to maintain integrity of the dosing valve assembly.
The problems associated with this high temperature environment have previously been addressed by water cooling the assembly. However, this requires specialized plumbing and systems that ultimately increase costs and reduce reliability. Geometrical configurations can increase or decrease the sensitivity to deposits.
Additionally, there are challenges relating to the quality of the spray when the volume of exhaust in the after-treatment process required is less, such as for smaller engine classes as used in privately owned vehicles and commuter vehicles typically less than four liters, and usually near two liters in engine displacement. Accordingly, there exists a need for a dosing valve which overcomes problematic spray quality due to a smaller mass flow rate of a reducing agent.